430
CHEMICAL ENGINEERING
marked position. Provision must be made for reversing the direction of the current from the battery through the galvanometer after this adjustment has been made. The figure does not show this or the switch for opening the circuit when the instrument is to be used for the low range.
Figure 9 shows the Leeds & Northrup instrument which is a modification of a design by W. P. White. With proper proportioning of the resistances, the slide wire may be set to read on a graduated scale the exact temperature required. The galvanometer G accordingly indicates the departure of the actual temperature from the temperature desired. It thus serves as a very convenient guide to the operator of a furnace who can see at a glance by how many degrees the temperature at any time differs from the temperature at which the furnace should be operated.
Temperature of the Cold Junctions of Thermocouples.—The electromotive force developed by a thermocouple depends upon the temperature of the cold
V • — VWMWMA--^ a B li L — VVWWAV — 1 c r, -i A/WVWVW — 'C
D V

FIG. 9.—Leeds & Northrup deflection potentiometer.
junctions as well as upon the temperature of the hot junction. For certain base metal couples having a nearly linear relation between temperature and electromotive force, the electromotive force is approximately proportional to the difference in temperatures of the hot and cold junctions. With such couples a change of 50°C. in the temperature of the cold junctions, unless corrected for, would result in an error of 50°C. in the final temperature measurement.
If a couple is calibrated with a cold-junction temperature of to°C. and is used with a cold-junction temperature of £VC. the true temperature of the hot junction is obtained by adding to the observed temperature the value (Z'o — to) K where K is a factor depending upon the particular couple employed and upon the temperature of the hot junction. The factor K varies from 1.5 to 0.3, but for rough work may be assumed 1.0 for base-metal couples and 0.5 for rare-metal couples. The following table gives the cold-junction factors for several different types of couple.
The corrections may be applied directly, without computing, by setting the pointer of the galvanometer to read the cold junction temperature on open circuit. The setting is made by turning the zero-adjustment screw of the indicator when the couple is disconnected. This method of correcting for the cold junction temperature is accurate but requires of course new settings whenever the temperature of the cold junction is altered. Indicators of the potentiometric type frequently have a movable slide on the temperature scale or an auxiliary dial (Cf. Bureau of Standards, Tech. Paper No. 170) which when set to the temperature of the cold junction gives perfect compensation at all temperatures of the hot junction. These two compensation methods also require new settings whenever the temperature of the cold junction is altered. With large and permanent installations the applying of corrections for the